Valve actuation in an internal combustion engine is required in order for the engine to produce positive power, as well as to produce engine braking. During positive power, intake valves may be opened to admit fuel and air into a cylinder for combustion. The exhaust valves may be opened to allow combustion gas to escape from the cylinder.
During engine braking, the exhaust valves may be selectively opened to convert, at least temporarily, an internal combustion engine into an air compressor. This air compressor effect may be accomplished by partially opening one or more exhaust valves near piston top dead center position for compression-release type braking, or by maintaining one or more exhaust valves in a partially open position for much or all of the piston motion for bleeder type braking. In doing so, the engine develops retarding horsepower to help slow the vehicle down. This can provide the operator increased control over the vehicle and substantially reduce wear on the service brakes of the vehicle. A properly designed and adjusted engine brake can develop retarding horsepower that is a substantial portion of the operating horsepower developed by the engine in positive power.
The braking power of an engine brake may be increased by selectively opening the exhaust and/or intake valves to carry out exhaust gas recirculation (EGR) in combination with engine braking. Exhaust gas recirculation denotes the process of channeling exhaust gas back into the engine cylinder after it is exhausted out of the cylinder. The recirculation may take place through the intake valve or the exhaust valve. When the exhaust valve is used, for example, the exhaust valve may be opened briefly near bottom dead center on the intake stroke of the piston. Opening of the exhaust valve at this time permits higher pressure exhaust gas from the exhaust manifold to recirculate back into the cylinder. The recirculation of exhaust gas increases the total gas mass in the cylinder at the time of the subsequent engine braking event, thereby increasing the braking effect realized.
For both positive power and engine braking applications, the engine cylinder intake and exhaust valves may be opened and closed by fixed profile cams in the engine, and more specifically by one or more fixed lobes which may be an integral part of each of the cams. The use of fixed profile cams makes it difficult to adjust the timings and/or amounts of engine valve lift needed to optimize valve opening times and lift for various engine operating conditions, such as different engine speeds.
One method of adjusting valve timing and lift, given a fixed cam profile, has been to incorporate a “lost motion” device in the valve train linkage between the valve and the cam. Lost motion is the term applied to a class of technical solutions for modifying the valve motion dictated by a cam profile with a variable length mechanical, hydraulic, or other linkage means. In a variable valve actuation lost motion system, a cam lobe may provide the “maximum” (longest dwell and greatest lift) motion needed for a full range of engine operating conditions. A variable length system may then be included in the valve train linkage, intermediate of the valve to be opened and the cam providing the maximum motion, to subtract or lose part or all of the motion imparted by the cam to the valve.
This variable length system (or lost motion system) may, when expanded fully, transmit all of the cam motion to the valve, and when contracted fully, transmit none or a partial amount of the cam motion to the valve. An example of such a system and method is provided in Vorih et al., U.S. Pat. No. 5,829,397 (Nov. 3, 1998), Hu, U.S. Pat. No. 6,125,828, and Hu U.S. Pat. No. 5,537,976, which are assigned to the same assignee as the present application, and which are incorporated herein by reference.
In some lost motion systems, an engine cam shaft may actuate a master piston which displaces fluid from its hydraulic chamber into a hydraulic chamber of a slave piston. The slave piston in turn acts on the engine valve to open it. The lost motion system may include a solenoid valve and a check valve in communication with a hydraulic circuit connected to the chambers of the master and slave pistons. The solenoid valve may be maintained in an open or closed position in order to retain hydraulic fluid in the circuit. As long as the hydraulic fluid is retained, the slave piston and the engine valve respond directly to the motion of the master piston, which in turn displaces hydraulic fluid in direct response to the motion of a cam. When the solenoid position is changed temporarily, the circuit may partially drain, and part or all of the hydraulic pressure generated by the master piston may be absorbed by the circuit rather than be applied to displace the slave piston.
Historically, lost motion systems, while beneficial in many aspects, have also been subject to many drawbacks. For example, the provision of hydraulic passages in various engine components, as is required in lost motion systems, may decrease the structural stiffness, and thus the effectiveness, accuracy, and lifespan of such components. The need for added components or components of increased size in order to accommodate a lost motion system may also increase valve train inertia to the point that it becomes problematic at high engine speeds. The use of hydraulics may also result in initial starting difficulties as the result of a lack of hydraulic fluid in the system. It may be particularly difficult to charge the system with hydraulic fluid when the fluid is cold and has a higher viscosity. Lost motion systems may also add complexity, cost, and space challenges due to the number of parts required. Furthermore, the need for rapid and repeated hydraulic fluid flow in prior art systems has also resulted in undesirable levels of parasitic loss and overheating of hydraulic fluid in the system.
Thus there is a need for, and the various embodiments of the present invention provide: improved structural stiffness compared to other lost motion systems that depend on displaced oil volumes to transmit motion; increased maximum valve closing velocities as compared to other lost motion systems; reduced cost and complexity due to the reduced number of high speed trigger valves and check valves required for the system; improved performance at initial start-up and decreased susceptibility to cold hydraulic fluid; decreased size and improved capability for integration into the cylinder head; reduced parasitic loss as compared with other lost motion systems; and improved hydraulic fluid temperature control.
The complexity of, and challenges posed by, lost motion systems may be increased by the need to incorporate an adequate fail-safe or “limp home” capability into such systems. In previous lost motion systems, a leaky hydraulic circuit could disable the master piston's ability to open its associated valve(s). If a large enough number of valves cannot be opened at all, the engine cannot be operated. Therefore, one valuable feature of various embodiments of the invention arises from the ability to provide a lost motion system which enables the engine to operate at some minimum level (i.e. at a limp home level) should the hydraulic circuit of such a system develop a leak. A limp home mode of operation may be provided by using a lost motion system which still transmits a portion of the can motion to the valve after the hydraulic circuit associated with the cam leaks or the control thereof is lost. In this manner the most extreme portions of a cam profile still can be used to get some valve actuation after control over the variable length of the lost motion system is lost and the system has contracted to a reduced length. The foregoing assumes, of course, that the lost motion system is constructed such that it will assume a contracted position should control over it be lost and that the valve train will provide the valve actuation necessary to operate the engine. In this manner the lost motion system may be designed to allow the engine to operate such that an operator can still “limp home” and make repairs.
A fundamental feature of lost motion systems is their ability to vary the length of the valve train. Not many lost motion systems, however, have utilized the high speed mechanisms that are required to rapidly vary the length of the lost motion system on a valve event-by-event basis. Lost motion systems accordingly have not been variable such that they may assume two functional lengths per cycle of the engine. The lost motion system that is the subject of this application is considerably advanced in comparison to other known systems due to its ability to provide variable valve actuation (VVA) on a valve event-by-event basis with each cycle of the engine. By using a high speed mechanism to vary the length of the lost motion system, more precise control may be attained over valve actuation, and accordingly optimal valve actuation may be attained for a wide range of engine operating conditions.
Applicants have determined that the lost motion system and method of the present invention may be particularly useful in engines requiring valve actuation for positive power, compression release engine braking, exhaust gas recirculation, cylinder flushing, and low speed torque increase. Typically, compression release and exhaust gas recirculation events involve much less valve lift than do positive-power-related valve events. Compression release and exhaust gas recirculation events may, however, require very high pressures and temperatures to occur in the engine. Accordingly, if left uncontrolled (which may occur with the failure of a lost motion system), compression release and exhaust gas recirculation could result in pressure or temperature damage to an engine at higher operating speeds. Therefore, it may be beneficial to have a lost motion system which is capable of providing control over positive power, compression release, and exhaust gas recirculation events, and which will provide only positive power or some low level of compression release and exhaust gas recirculation valve events, should the lost motion system fail. It may also be beneficial to provide a lost motion system capable of providing post main exhaust valve events which may be used to achieve cylinder flushing and low speed torque increases.
An example of a lost motion system and method used to obtain retarding and exhaust gas recirculation is provided by the Gobert, U.S. Pat. No. 5,146,890 (Sep. 15, 1992) for a Method And A Device For Engine Braking A Four Stroke Internal Combustion Engine, assigned to AB Volvo, and incorporated herein by reference. Gobert discloses a method of conducting exhaust gas recirculation by placing the cylinder in communication with the exhaust system during the first part of the compression stroke and optionally also during the latter part of the inlet stroke. Gobert uses a lost motion system to enable and disable retarding and exhaust gas recirculation, but such system is not variable within an engine cycle.
In view of the foregoing, there is a significant need for a system and method of controlling lost motion which: (i) optimizes engine operation under various engine operating conditions; (ii) provides precise control of lost motion; (iii) provides acceptable limp home and engine start-up capability; and (iv) provides for high speed variation of the length of a lost motion system. The lost motion system that is the subject of this application meets these needs, as well as others.
As noted above, one constraint on the use of lost motion systems arises from the addition of bulk in the engine compartment. Known systems for providing lost motion valve actuation have tended to be non-integrated devices which add considerable bulk to the valve train. As vehicle dimensions have decreased, so have engine compartment sizes. Accordingly, there is a need for a less bulky lost motion system, and in particular for a system which is compact and has a relatively low profile.
Furthermore, there is a need for low profile lost motion systems capable of varying valve actuation responsive to engine and ambient conditions. Variable actuation of intake and exhaust valves in an internal combustion engine may be useful for all potential valve events (positive power and engine braking). When the engine is in positive power mode, variation of the opening and closing times of intake and exhaust valves may be used in an attempt to optimize fuel efficiency, power, exhaust cleanliness, exhaust noise, etc., for particular engine and ambient conditions. During engine braking, variable valve actuation may enhance braking power and decrease engine stress and noise by modifying valve actuation as a function of engine and ambient conditions.
In an attempt to develop a functional and robust variable valve actuation system that is useful for both positive power and engine braking applications, Applicants have had to solve several design challenges. These design challenges have resulted in the development of sub-systems that not only allow the subject system to work effectively, but which may also be useful in other variable valve actuation systems.
For example, engine valves are required to open and close very quickly, therefore the valve spring is typically very stiff. When the valve closes, it may impact the valve seat with such force that it eventually erodes the valve or the valve seat, or even cracks or breaks the valve. In mechanical valve actuation systems that use a valve lifter to follow a cam profile, the cam lobe shape provides built-in valve-closing velocity control. In common rail hydraulically actuated valve assemblies, however, there is no cam to self-dampen the closing velocity of an engine valve. Likewise, in hydraulic lost motion systems such as the present ones, a rapid draining of fluid from the hydraulic circuit may allow an engine valve to “free fall” and seat at an unacceptably high velocity.
The system that is the subject of this application, being a lost motion system, presents valve seating challenges. The variable valve actuation capability of the present system may result in the closing of an engine valve at an earlier time than that provided by the cam profile. This earlier closing may be carried out by rapidly releasing hydraulic fluid (to an accumulator in the preferred embodiment) in the lost motion system. In such instances engine valve seating control is required because the rate of closing the valve is governed by the hydraulic flow to the accumulator instead of by the fixed cam profile. Engine valve seating control may also be required for applications (e.g. centered lift) in which the engine valve seating occurs on a high velocity region of the cam.
Applicants approached the valve seating challenge with the understanding that valve seating velocity should be less than approximately 0.4 msec. Absent steps to control valve seating velocity, however, the valves could seat at a velocity that is an order of magnitude greater. Applicants also determined that valve seating control preferably should be designed to function when the closing valve gets within 0.5 to 0.75 mm of the valve seat. The combination of valve thermal growth, valve wear, and tolerance stack-up can exceed 0.75 mm, resulting in the complete absence of seating velocity control or in an exceedingly long seating event if measures are not taken to adjust the lash of the valve seating control to account for such variations. It is also assumed that, preferably, valve seating control should not significantly reduce initial engine valve opening rate, and valve seating control should be capable of operating over a wide range of valve closing velocities and oil viscosities.
Existing devices used to control valve seating velocity may use hydraulic fluid flow restriction to produce pressure that acts on an area of the slave piston to develop a force to slow the slave piston and reduce seating velocity. The area on which the pressure acts may be very small in such devices which in turn requires that the pressure opposing the valve return spring be high, and the controlling flow rate be low. Low controlling flow rates result in an increased sensitivity to leakage. In addition, these devices may restrict the hydraulic fluid flow that produces valve opening.
In view of the foregoing there is a need for a valve catch sub-system for valve seating control that provides fine control of hydraulic fluid flow through the sub-system. There is also a need for a sub-system that does not adversely affect hydraulic fluid flow for valve opening and which is less susceptible to dimensional tolerances affecting leakage. In particular, there is a need for valve seating that is improved by a flow control that becomes more restrictive as the valve approaches the seat.
There is also a need for a valve catch that adjusts for lash differences between the engine valve and the valve catch. Although most variable valve actuation (VVA) systems are inherently self lash adjusting, valve seating control is not. Systems that do not need manual adjustment, either initially or as the system ages, are desirable. Previous valve seating control mechanisms have required a manual lash adjustment or a separate set of lash adjustment hardware. The design of a conventional hydraulic lash adjustor capable of transmitting compression-release braking loads would be challenging due to structural and compliance requirements.
The valve catch embodiment(s) of the present invention meet the aforementioned needs and provide other benefits as well. The valve catch embodiment(s) disclosed herein provide acceptable engine valve seating velocity in a VVA system, such as a lost motion or common rail system. For a lost motion WA system, engine valve seating control is provided for early engine valve closing, where the rate of closing is governed by the hydraulic flow from the control piston to the accumulator as opposed to a cam profile. Engine valve seating control also may be provided for a high velocity region of the cam. The lash adjusting portion of this mechanism provides an additional amount of seating control for the last few hundredths of a millimeter of valve closing.
The valve catch embodiment(s) of the present invention includes a variable flow area in the sub-system plunger. The valve catch embodiment(s) of the invention may also be designed to have relatively high flow rates, large orifices, and utilize small pressure drops. The valve catch embodiment(s) of the present invention may also experience reduced peak valve catch pressure as compared with some known valve catch systems. Furthermore, the variable flow restriction design of the valve catch embodiment(s) of the present invention is expected to be more robust than the constant flow restriction design with respect to engine valve velocity at the point of valve catch engagement and oil temperature and aeration control. Variable flow restriction may allow the displacement at the point of valve catch/slave piston engagement to be reduced, so that the valve catch has less undesired effect on the breathing of the engine.
Furthermore, Applicants implementation of a variable valve actuation system using lost motion hydraulic principles may require a sub-system for effecting initial start up of the system. An initial start mechanism (ISM) may be required to (i) accelerate the process of charging the subject lost motion system with hydraulic fluid, and/or (ii) permit actuation of the engine valve until such time as the subject system is fully charged with hydraulic fluid. Absent such a system, starting and/or smooth operation of the engine could be delayed due to the inaction of the engine valves until there is sufficient hydraulic fluid in the system to produce the desired valve motions. An added advantage of such a system is that it may provide a limp-home mode of operation for the engine as well in the event that the system is incapable of being charged with hydraulic fluid. Therefore, there is a need for a sub-system that provides valve actuation between the initial cranking of an engine and the charging of the variable valve actuation system with hydraulic fluid.
Still other advancements that may be required for operation of the subject system include an accumulator sub-system. In order to broaden the range of possible valve actuations that may be produced with the subject system, it may be beneficial to improve the rate at which the accumulator can absorb fluid and the rate at which it can supply fluid for re-fill operations. Improvement of this response time may permit more rapid variation of the motion of the engine valves in the system and may limit the loss of cam follow during periods of hydraulic fluid flow from the accumulator to the high-pressure hydraulic circuit. Accordingly, there is a need for a system accumulator with improved response time.
A basic method of improving accumulator response time is to increase the strength of the spring biasing the accumulator piston into its refill position. However, accumulator spring force cannot be increased indefinitely without incurring associated costs. For example, the accumulator spring force should be limited relative to the engine valve spring force so as to avoid engine valve float. In turn, the engine valve spring force may be limited by spring envelope constraints and the need to minimize parasitic loss of the VVA system.
Furthermore, the accumulator design would ideally prevent the high-pressure circuit pressure from dropping below ambient or the accumulator piston from bottoming out in its bore, because these situations could cause cavitation and evolution of dissolved air in the oil. This problem may be particularly troublesome during an early engine valve closing event, where oil must quickly flow to the accumulator to affect the early closing and then flow back to the high-pressure circuit when the engine valve seats or valve catch engages.
Despite all of the foregoing design challenges, Applicants have designed a compact and efficient accumulator system that provides improved response time. Applicants have designed a relatively low pressure accumulator system which provides improved performance as the result of synergy attributable to the combination of a low restriction trigger valve, shorter and larger fluid passages between the system elements, use of fewer or no check valves, larger yet low inertia accumulator pistons, reduced accumulator piston travel, and a gallery arrangement of multiple accumulators in common hydraulic communication.
Control feature advancements also appear to be desirable in view of the capabilities of the subject VVA system. For example, in some embodiments of the present invention, each of the engine valves in the subject system may be independently turned “on” or “off” for a prolonged period. Accordingly, there is a need for advanced control features, such as cylinder cut-out capability, which may reduce fuel consumption by only activating individual engine valves or engine valves associated with individual cylinders, on an as needed basis.
Control over cylinder cut-out necessarily requires active control over cylinder re-start. Assuming the cylinder cut-out is controlled in response to engine load (the lower the load, the less cylinders needed for power), then cylinder re-start must also be provided responsive to increasing engine load. Embodiments of the present invention provide for such active control over cylinder re-start, as well as cylinder cut-out.
The use of hydraulic actuation also may necessitate control features that modify the timing of hydraulic actuation based on the viscosity of the hydraulic fluid in the system. Typically, the viscosity of hydraulic fluid, such as engine oil, lowers as it increases in temperature. As viscosity lowers, the response time for hydraulic actuation involving the fluid may decrease. Because the temperature of the hydraulic fluid used in connection with the various embodiments of the present invention may vary by more than 100 degrees Celsius, there is a need to adjust the timing of some hydraulic actuation events based on the temperature and/or viscosity of the hydraulic fluid. Various embodiments of the present invention provide for modification of hydraulic actuation based on the temperature and/or viscosity of the hydraulic fluid used for such actuation.
Others have attempted to provide for the modification of valve actuation systems. U.S. Pat. No. 5,423,302 to Glassey discloses a fuel injection control system having actuating fluid viscosity feedback using several sensors including a crankshaft angular speed sensor, an engine coolant temperature sensor, and a voltage sensor. U.S. Pat. No. 5,411,003 to Eberhard et al. (“Eberhard”) discloses a viscosity sensitive auxiliary circuit for a hydromechanical control valve for timing the control of a tappet system. Eberhard utilizes a pressure divider chamber to influence timing control. U.S. Pat. No. 4,889,085 to Yagi et al. discloses a valve operating device for an internal combustion engine that utilizes a damper chamber in connection with a restriction mechanism. Some of these inventions attempt to compensate for increased viscosity by modifying the flow of working fluid, rather than the timing of the operation of the valves themselves. In addition, many of these devices are complex and difficult to maintain. Accordingly, there remains a need for a method and apparatus for modifying the opening and closing of engine valves based on an engine fluid temperature and/or viscosity that is accurate, easy to implement, cost effective, and easy to calibrate by the user.
As may be evident, the embodiments of the present invention disclosed herein may be particularly useful in a wide variety of internal combustion engines. Such engines are often considered to emit undesirably high levels of noise. Accordingly, various embodiments of the invention may also incorporate control features which tend to reduce the level of noise produced by such engines, both during positive power and during engine braking.